Synthetic quinolone antibacterial agents having a quinoloine skeleton have so far been widely studied.
For example, it was found that the antibacterial activity of a synthetic antibacterial agent is remarkably increased by introducing a fluorine group into the 6-position of quinolone skeleton thereof. Fluorine atom at the 6-position is considered to be a structural characteristic indispensable for synthetic quinolone antibacterial agents of the second generation or, in other words, new synthetic quinolone antibacterial agents. Pyrrolidine ring, piperazine ring or the like introduced through C—N bond is also considered to be an indispensable structural characteristic because a substituent at the 7-position of the quinolone structure exerts a considerable influence on the antibacterial activity, internal dynamics and toxicity.
Synthetic quinolone antibacterial agents have so far clinically widely been used as excellent agents having a high antibacterial activity. However, recently, it has become possible to separate many multiple drug-resistant bacteria also resistant to the synthetic quinolone antibacterial agents. However, fluorine atom at the 6-position of the quinolone skeleton and the substituent introduced through C—N bond at the 7-position of the quinolone skeleton, which are indispensable in the structure of the new synthetic quinolone antibacterial agents, are considered to be the causes for the side effects of the new synthetic quinolone antibacterial agents on the central nervous system. Those problems of the multiple drug-resistant bacteria and the side effects are mostly due to the structural characteristics of the new synthetic quinolone antibacterial agents. Under these circumstances, it is demanded to develop a drug having a skeleton different from the quinolone skeleton of the current antimicrobial agents and also a high antimicrobial activity.